Serotonin (5-HT) plays a key role in the development of craniofacial structures as well as in the establishment of their representations in the cerebral cortex. The effects of 5-HT on craniofacial morphogenesis are thought to be mediated primarily by the high-affinity 5-HT transporter (SERT). In contrast, the mechanisms underlying 5-Ht's ability to profoundly alter the cortical representations of craniofacial structures are not all understood. The goal of the proposed research is to understand the mechanism(s) underlying the ability of 5-HT to alter somatosensory thalamocortical axons. Our previous results have shown that developing thalamocortical axons transiently express both 5-HT1B receptors and SERT, and that elevating or decreasing 5-HT levels in the developing cortex markedly alters the organization of thalamocortical axons. Our previous results have shown that developing thalamocortical axons transiently express both 5-HT1B receptors and SERT, and that elevating or decreasing 5-HT levels in the developing cortex markedly alters the organization of thalamocortical afferents related to the mystacial vibrissae. Our working hypothesis is that 5-HT acts through the 5-HT1B receptor and/or SERT to produce these effects. The experiments comprising this application use multiple parallel approaches to test this hypothesis. Two in vivo experiments evaluate the role of the 5-HT1B receptor on thalamocortical development. First, a highly specific agonist of this receptor, 5-nonyloxytryptamine (NNT), will be applied to cortex in developing rats and its effects on thalamocortical organization assessed by labeling the axons of central posteromedial thalamic neurons with Di- I. Second, cortical 5-HT levels will be raised via clorgyline treatment in transgenic mice effects of elevated 5-HT will be blunted in the knockout mice. Evaluation of the effects of NNT on the organization of thalamocortical axon terminals in rats and elevated 5-HT in 5-HT1B knockout mice in vivo will be paralleled by experiments in which the effects on NNT on axonal outgrowth by thalamocortical cells from rats and 5-HT on neurons from 5-HT1B knockout mice are assessed in vitro using methods already established in the laboratory. The same approaches will be used to evaluate the role of SERT in 5-HT modulation of thalamocortical development. In in vivo experiments, 5-HT levels will be raised using clorgyline in the cortices of rats in which uptake of this amine is blocked by local application of fluoxetine, and in transgenic mice lacking SERT. If SERT mediates the effects of 5-HT levels should not be seen. These in vivo experiments will again be paralleled by in vitro studies in which the effects of 5-HT upon neurite outgrowth are evaluated in thalamic cells from rats in the presence of fluoxetine and in thalamic neurons from transgenic mice lacking SERT.